You can diet the whole week and have a “cheat” day (Shabbos).
Do what others suggested and try not to drink pop. Try alternatives such as, water with citrus in it (I find lemons work best, but limes and even oranges also work fine). Try to limit portion sizes. Eat the cake but take a smaller size.
Shabbos is meant for us to enjoy, not gorge ourselves. Try not to eat the delicious food when you are full. Also eat slower and savor each bite. Think how much eating the food enhances your Shabbos.